1. Field of the Invention
The present invention relates to hydraulic power systems using pumps with pressure compensators.
2. Description of Related Art
In aircraft hydraulic systems, hydraulic pressure is maintained at a constant magnitude under changing flow demands by using pumps with pressure compensation mechanisms. For each pump, as hydraulic system flow demands change, the compensator adjusts the pump displacement by sensing and responding to the system pressure. If the system pressure drops, the compensator increases the pump displacement, thereby increasing flow and boosting the system pressure. If the system pressure increases, the compensator decreases the pump displacement, thereby decreasing flow and lowering the system pressure.
In most aircraft, there is usually no way to correct a failed pump. In pump failure situations, the failed pump is ignored and a backup pump is used. However, pressure relief valves are utilized in aircraft hydraulic systems to reduce high system pressures that result from pump compensators that fail and remain stuck in the maximum flow position. When pump compensators fail and remain in the maximum flow position, excessive heat is generated by the high flow rates through the hydraulic system. As a result, heat exchangers must be added to the hydraulic system to dissipate the excess heat.
There are basically two methods used in aircraft hydraulic system design to prevent system overheating as a result of hydraulic pump compensator failures. One method is to oversize the hydraulic system heat exchanger capacity by about 40–50% to account for the additional heat resulting from the failure. This method requires additional space on the aircraft and adds a significant amount of weight to the aircraft.
The other method is to install a solenoid operated bypass valve or shut-off valve that allows the operator to manually isolate the pump from the hydraulic system. With a bypass valve, the solenoid actuates a spool that connects the outlet to the inlet. With a shut-off valve, the solenoid pushes a spool that blocks the outlet completely. Once the solenoid operated bypass valve or shut-off valve is activated, all hydraulic power from that system is lost. Solenoid operated bypass valves and shut-off valves are relatively unreliable, and require an external electrical power source. This increases their probability of failure. In addition, this method can result in the failure of a hydraulic system as a result of an electrical short.
Referring to FIGS. 1 and 2 in the drawings, a prior-art variable displacement pump 11 having a pressure compensator valve, also known as a flat cut-off pump, is illustrated. Pump 11 has a case 13, a drive shaft 15, a rotating block 17 driven by drive shaft 15, pistons 19 and 21, and a pivoting pump yoke 23. Pump yoke 23 is spring biased against a yoke actuating piston 25 by a yoke spring 27. Yoke actuating piston 25 is actuated by a compensator valve 29. The trigger pressure of compensator valve 29 is controlled by a compensator valve spring 31 and a pressure adjustment screw 33. Actuation of yoke actuating piston 25 causes pump yoke to pivot about a pivot pin 29, thereby adjusting the stoke displacement of pistons 19 and 21. As is shown in FIG. 2, pump yoke 23 pivots between a minimum stroke position indicated by dashed lines, and a maximum stroke position indicated by solid lines.
If the outlet pressure exceeds the trigger pressure of compensator valve 29, compensator valve 29 opens causing an increase in the pressure on yoke actuating piston 25. Actuation of yoke actuating piston 25 forces pump yoke 23 to pivot about pivot pin 29 against yoke spring 27 into a position in which the stoke displacement of pistons 19 and 21 is reduced. The reduction in the stoke displacement of pistons 19 and 21 reduces the outlet pressure.
A specific compensator mechanism failure mode that must be considered when designing a hydraulic system is when the compensator valve sticks in the maximum displacement position. Under this type of failure, the pump flow exceeds system demand, resulting in the system pressure exceeding the allowable design limit. For most aircraft hydraulic systems, the allowable design limit pressure is 50% higher than the normal system pressure. To prevent damage to the hydraulic system as a result of the failure of a compensator valve, pressure relief valves are incorporated into the hydraulic system to ensure that the system pressure does not exceed safe values.
To ensure that the pressure relief valve does not open unless the pump compensator fails, the opening pressure of the relief valve is usually set 20–30% higher than the normal system operating pressure. For example, in an aircraft hydraulic system having a normal system operating pressure of about 3,000 psi, the design limit pressure would be about 4,500 psi, and the pressure relief valve would be designed to open at about 3,600–3,900 psi.
Although the pressure relief valve protects the hydraulic system from damage due to over pressurization, relief valve operation can induce a second equally critical problem: hydraulic system overheating. As a byproduct of the normal work performed by the pump pushing fluid through the hydraulic system, heat is generated. The larger the flow or higher the system pressure, the greater the heat generated. To address this problem, heat exchanges, or radiators, are incorporated into the hydraulic system to dissipate the excess heat.
Referring now to FIG. 3 in the drawings, a schematic of a typical prior-art hydraulic system 51 is illustrated. Hydraulic system 51 is representative of a wide variety of hydraulic systems, not just aircraft hydraulic systems. Hydraulic system 51 includes a hydraulic pump 53, a hydraulic reservoir 55, a hydraulic actuator 57, a pressure relief valve 59, and a heat exchanger 61.
Referring now to FIG. 4 in the drawings, a schematic of another typical prior-art hydraulic system 71 is illustrated. Hydraulic system 71 is also representative of a wide variety of hydraulic systems, not just aircraft hydraulic systems. Hydraulic system 71 includes a hydraulic pump 73, a hydraulic reservoir 75, a hydraulic actuator 77, a pressure relief valve 79, and a heat exchanger 81. Hydraulic system 71 also includes a solenoid operated bypass valve 83 for isolating hydraulic system 71 by connecting the inlet port to the outlet port.
The size of the heat exchanger required for a given hydraulic system is normally based on the average pump flow at the normal system operating pressure. However, following a pump compensator failure and resultant opening of a pressure relief valve, system pressure typically increases by 20–30%. Therefore, to prevent the hydraulic system from overheating following a pump compensation failure, either the heat exchanger capacity must be greatly increased, or a device must be incorporated to relieve system pressure to a level below normal operating pressure.
The current methods of preventing hydraulic systems from overheating following pump compensation failures do not adequately solve the problem. Solenoid operated bypass valves or shut-off valves are unreliable, require an electrical power source, and add weight to the system. Oversizing the heat exchangers is expensive, requires additional space, and adds weight to the system. Thus, although these methods represent great strides in the area of hydraulic power systems, many shortcomings remain.